Organic molecules

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Everything Science www.everythingscience.co.za
4. Organic Molecules
Chemistry
Grade 12

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Introduction
Organic chemistry is the branch of chemistry that deals with organic molecules. An organic
molecule is one that contains carbon atoms (generally bonded to other carbon atoms as well
as hydrogen atoms).
All living organisms contain carbon. Plants use sunlight to convert carbon dioxide in the air
into organic compounds through the process of photosynthesis. Animals and other organisms
then feed on plants to obtain their own organic compounds. Fossil fuels are another important
source of carbon.
It is the unique properties of the carbon atom that give organic compounds certain properties.
The carbon atom has four valence electrons, so it can bond with many other atoms, often
resulting in long chain structures. It also forms mostly covalent bonds with the atoms that it
bonds to.

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Representing organic molecules
An organic compound can be represented in different ways, using its molecular formula,
structural formula, semi-structural formula or condensed structural formula.
Structural formula
Semi-structural formula
CH3 CH(CH3)CH3
C4 H10
Condensed structural formula
Molecular formula

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Functional groups
A functional group is a particular group of atoms within a molecule, which give it certain reaction
characteristics. Organic compounds can be grouped according to their functional group.

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Saturated and unsaturated molecules
A molecule is said to be saturated if it contains the maximum possible number of hydrogen
(or other) atoms for that molecule. The alkanes are all saturated compounds.
A molecule is unsaturated if it does not contain the maximum number of hydrogen atoms for
that molecule. The alkenes and alkynes are examples of unsaturated molecules. If a double
or triple bond is broken, more hydrogen (or other) atoms can be added to the molecule.

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Hydrocarbons
The hydrocarbons are organic compounds that contain only carbon and hydrogen. They can
be further divided into the alkanes, alkenes and alkynes, based on the type of bonds between
the carbon atoms.
The alkanes have only single bonds between their carbon atoms and are unreactive. The
general formula for the alkanes is Cn
H2n+2
The alkenes have at least one double bond between two of their carbon atoms. The general
formula for the alkenes is Cn
H2n
The alkynes have at least one triple bond between two of their carbon atoms. They are both
more reactive than the alkanes. The general formula for the alkynes is Cn
H2n-2
Alkyl halides (haloalkanes) are hydrocarbons with one hydrogen atom replaced by a halogen
atom (F, Cl, Br or I). The halogen is called a substituent (an atom or group of atoms bonded to
a carbon chain is termed a substituent).

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Alcohols
An alcohol is an organic compound that contains a hydroxyl group (−OH) The general formula
for the alcohols is Cn
H2n+1
OH
The alcohols have a number of different uses including their use as a solvent, for medicinal
purposes and in alcoholic drinks.
The alcohols share a number of properties because of the hydroxyl group. The hydroxyl group
affects the solubility of the alcohols (in water). Those with shorter carbon chains are generally
more soluble, and those with longer chains are less soluble. The strong hydrogen bond
between the hydrogen and oxygen atoms in the hydroxyl group gives alcohols a higher
melting point and boiling point than the hydrocarbons.

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Carbonyl containing compounds
A carbonyl group consists of an oxygen atom that is double-bonded to a carbon atom.
In a ketone the carbonyl group is in the middle of the carbon chain. In an aldehyde the
carbonyl group is at the end of the chain. The general formula for both aldehydes and
ketones is Cn
H2n
O.
The carboxylic acids are organic acids that contain a carboxyl group with the formula
−COOH. In a carboxyl group a carbon atom is double-bonded to an oxygen atom, and it is
also bonded to a hydroxyl group. The general formula for the carboxylic acids is
Cn
H2n+1
COOH.
The carboxylic acids have weak acidic properties because the hydrogen atom is able to
dissociate from the carboxyl group.
An ester is formed when an alcohol reacts with a carboxylic acid. Esters contain a carbonyl
functional group as well as an oxygen atom bonded to the carbonyl carbon atom and another
carbon atom.

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Isomers
If two compounds are isomers, it means that they have the same molecular formulae but
different structural formulae.
Positional isomers:
Functional isomers:

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Naming organic compounds
Organic compounds are named according to their functional group and its position in the
molecule, the number of carbon atoms in the molecule and the position of any double and
triple bonds. The IUPAC rules for nomenclature are used in the naming of organic molecules.

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Naming organic compounds contd.
1. Recognise the functional group in the compound. This will determine the suffix of the name.
2. Find the longest continuous carbon chain that contains the functional group (it won’t always
be a straight chain) and count the number of carbon atoms in this chain. This number will
determine the prefix (the beginning) of the compound’s name.
3. Number the carbons in the longest carbon chain (Important: If the molecule is not an alkane
(i.e. has a functional group) you need to start numbering so that the functional group is on the
carbon with the lowest possible number). Start with the carbon at the end closest to the
functional group.
4. Look for any branched groups.
5.Combine the elements of the name into a single word in the following order: branched
groups/halogen atoms in alphabetical order (ignoring prefixes), prefix of main chain, name
ending according to the functional group and its position on the longest carbon chain.

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Properties of hydrocarbons
Many of the properties of the hydrocarbons are determined by their molecular structure, the
bonds between atoms and molecules, and their surface area.
The melting points and boiling points of the hydrocarbons increases as their number of
carbon atoms increases.
The molecular mass of the hydrocarbons determines whether they will be in the gaseous,
liquid or solid phase at specific temperatures.
The weaker the intermolecular forces between molecules the more volatile and the higher the
vapour pressure of that compound. Haloalkanes become less volatile the more halogen
atoms they contain. This is due to increased intermolecular forces.

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Reactions
Three types of reactions that occur are addition, elimination and substitution.
An addition reaction occurs when two or more reactants combine to form a single product.
This product will contain all the atoms that were present in the reactants. Addition reactions
occur with unsaturated compounds. Hydrohalogenation, halogenation, hydration,
hydrogenation and polymerisation reactions are all addition reactions.
An elimination reaction occurs when a reactant is broken up into two products. Elimination
reactions occur with saturated compounds. Dehydrohalogenation and dehydration of an
alcohol are two examples of elimination reactions.
A substitution reaction occurs when an exchange of elements in the reactants takes place.
The initial reactants are transformed or swopped around to give a final product. Formation of
haloalkanes and hydrolysis are two examples of substitution reactions.
A+B→C
A→B+C
AB+CD→AD+BC

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Polymers and plastics
A polymer is a macromolecule that is made up of many repeating structural units called
monomers, which are joined by covalent bonds. Polymers that contain carbon atoms in the
main chain are called organic polymers.
Organic polymers can be divided into natural organic polymers (e.g. natural rubber) or
synthetic organic polymers (e.g. polystyrene).
Polymers form through a process called polymerisation. Two examples of polymerisation
reactions are addition and condensation reactions.
An addition polymerisation reaction occurs when unsaturated monomers (e.g. alkenes) are
added to each other one by one. The breaking of a double bond between carbon atoms in the
monomer means that a bond can form with the next monomer.
In a condensation polymerisation reaction a molecule of water is released as a product of
the reaction. The water molecule is made up of atoms that have been lost from each of the
monomers.

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Properties and types of polymers
The chemical properties of polymers (e.g. ability to withstand stress and melting point) are
determined by the types of atoms in the polymer, and by the strength of the bonds between
adjacent polymer chains. The stronger the bonds, the greater the strength of the polymer, and the
higher its melting point.
Polyethene, polypropene, polyvinyl chloride, polyvinyl acetate and polystyrene are all polymers
formed through addition polymerisation.
Polyethylene terephthalate and polylactic acid are polymers formed through condensation
polymerisation.

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Organic molecules

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